1. Field of the Invention
The present invention relates to a bearing with integrated rotation sensor which may be used in electric motors and other various equipments.
2. Description of the Prior Art
The bearing with integrated rotation sensor of a kind referred to above is largely used in an application for detection of, for example, the rotational speed (the number of revolutions) or the direction of rotation and, in such case, the sensor output system is a two phase system capable of outputting an A phase signal and a B phase signal. The two phase signals are in the form of an incremental pulse signal and the rotational speed can be detected in reference to one of those two phase signals. Also, a phase difference of 90° is given between the A phase signal and the B phase signal and the phase difference can be utilized to detect the direction of rotation.
In the sensor output system of the kind discussed above, detection of the rotation angle requires an addition of a zero point position signal (a Z phase signal) descriptive of the position of origin with respect to the direction of rotation, other than the above discussed two phase signals. In such case, it is a general practice to detect the rotation angle from the position of origin in reference to the relation between one of the A and B phase signals and the Z phase signal.
As a magnetic encoder that is used in the rotation sensor having such a capability of detecting the rotation, the Japanese Laid-open Patent Publication No. 2004-101312, published Apr. 2, 2004, discloses such a type as shown in FIG. 10. In this known magnetic encoder 17 shown therein, a first to-be-detected member 17A necessary to obtain the A phase signal and the B phase signal and a second to-be-detected member 17B necessary to obtain the Z phase signal are arranged in an outer periphery of a cylindrical metal core 20 in side-by-side relation in a direction axially of the metal core 20. The first to-be-detected member 17A is in the form of a ring-shaped magnet having a plurality of alternating magnetic poles N and S deployed in a direction circumferentially of the metal core 20 and having a uniform thickness in a radial direction over the entire circumference thereof. On the other hand, the second to-be-detected member 17B is in the form of a magnet connected with the first to-be-detected member 17A and including a thick walled portion of a thickness equal to that of the first to-be-detected member 17A and having one of the alternate magnetic poles, for example, a magnetic pole S. The remaining circumferential portion of the second to-be-detected member 17B is a thin walled portion of a thickness smaller than that of the thick walled portion. By so constructing, when works to magnetize the first and second to-be-detected members 17A and 17B to form the respective magnetic poles are carried out simultaneously, the plurality of the alternating magnetic poles can be formed in the first to-be-detected member 16A so as to deploy in the circumferential direction of the metal core 20 and, at the same time, the magnetic pole can be formed only in the thick walled portion of the second to-be-detected member 17B. The thick walled portion of the second to-be-detected member 17B may have a magnetic north (N) pole on each side of the magnetic south (S) pole in a direction circumferentially of the metal core 20.
The known magnetic encoder 17 of the structure described above is fixed on, for example, an outer periphery of an inner race which forms a rotating ring of a bearing, and, as shown in FIG. 11, an outer race which forms a stationary ring of the bearing is provided with two magnetic sensors 18A and 18B held in face-to-face relation with the first to-be-detected member 17A for the detection of an A phase signal and a B phase signal, respectively, and also with a magnetic sensor 18C held in face-to-face relation with the second to-be-detected member 17B for the detection of a Z phase signal. The two magnetic sensors 18A and 18B are arranged spaced a distance from each other in the circumferential direction so that respective output signals from those magnetic sensors 18A and 18B can have a phase difference of, for example, 90°.
In the bearing with integrated rotation sensor so constructed as hereinabove described, as the bearing inner race rotates, the two magnetic sensors 18A and 18B for detecting the magnetic poles N and S in the first to-be-detected member 17A output the A phase signal and the B phase signal, which are offset a phase difference of 90° relative to each other, respectively. Also, the magnetic sensor 18C for the detection of the Z phase signal operates in such a manner that each time the bearing inner race undergoes one complete rotation, it will not detect any magnetism in a region of rotation confronting the thin walled portion of the second to-be-detected member 17B as shown in FIG. 11A, but will detect a magnetism in a region of rotation confronting the thick walled portion as shown in FIG. 11B to thereby output the Z phase signal at one time. In this way, the rotational speed, the direction of rotation and the position of origin can be detected.
In the magnetic encoder 17 of the structure hereinabove described, since a circumferential portion of the second to-be-detected member 17 other than the thick walled portion is formed as a thin walled portion having a weak magnetic force, the magnetic sensor 18C for the detection of the Z phase signal is not affected by the magnetic field emanating from the magnetic encoder 17 when the magnetic sensor 18C is brought in position to confront the thin walled portion of the second to-be-detected member 17. Where the bearing with integrated rotation sensor utilizing this magnetic encoder 17 is used as built in, for example, an electrical drive motor, and when the magnetic field leaking from the electric drive motor acts on the bearing with integrated rotation sensor, the magnetic field entering the inside of the bearing may extend through the magnetic encoder 17. In such case, since the thin walled portion in the second to-be-detected member 17B does not work as a magnetic encoder sufficiently as hereinbefore described, it will be dominated by only the magnetic field entering from the outside and will be held in a state as if the magnetic field is generated from the magnetic encoder 17. Under these circumstances, the magnetic sensor 18C for the detection of the Z phase signal may detects a magnetism, which has entered from the outside, at the thin walled portion in the second to-be-detected member 17B and, therefore, it will erroneously output a plurality of pseudo Z phase signals for each complete rotation, instead of outputting one Z phase signal per one complete rotation.